Automatic power transmissions used in modern vehicles typically include a multi-function hydrodynamic device commonly referred to as a torque converter. A torque converter is a fluid coupling device that automatically disengages a rotating engine crankshaft from a transmission input shaft during vehicle idling conditions to enable the vehicle to stop and/or to shift gears without stalling. Additionally, a torque converter may be used as a torque multiplier for multiplying engine torque in a lower vehicle speed range until the vehicle speed nearly matches engine speed.
Within a torque converter, a pump, a turbine, and a stator combine to enable efficient fluid coupling of the rotating engine and transmission shafts. The pump is the “driving” member of the torque converter assembly giving impetus to a stream of hydraulic fluid, and is connected to the engine crankshaft to rotate in unison therewith for accelerating a supply of hydraulic fluid. The accelerated fluid is directed into the turbine, or the “driven” member of the torque converter assembly. The turbine is typically splined or otherwise directly attached to a transmission input shaft, and thus converts the energy imparted by the fluid stream into useable mechanical energy. This energy is then transferred to the transmission input shaft to propel the vehicle. Finally, the stator is a stationary member redirecting the accelerated fluid stream between the pump and turbine. The stator is connected to a fixed reaction shaft through a one-way clutch to permit the stator to free-wheel when torque multiplication is no longer possible.
Torque converters are designed to slip at low vehicle speeds in order to enable the transmission to rotate at a slower rate relative to the coupled engine. The slip rate gradually diminishes as the vehicle is accelerated up to a threshold speed. Fuel efficiency may be reduced as a result of such slip, however, as only a portion of available engine power is utilized while operating under slip conditions. To minimize slip, torque converters may be configured with a torque converter clutch, also known as a lock-up clutch, which mechanically joins the separate rotating turbine and pump portions of the torque converter. Engagement of a torque converter clutch is generally determined and applied in relation to the throttle/gas position and vehicle speed, locking the turbine and pump above a threshold engine speed.